Thermoformed plastic containers are well known as inexpensive and highly customizable containers for the sale of a wide variety of products, everything from cell phones to deli meats. Thermoformed plastic containers are typically transparent and rigid, so they can give a consumer the ability to examine a product closely. They can be made tamper-resistant, to reduce the risk that the product could be damaged or contaminated. They are typically lightweight, and can be manufactured in a wide variety of shapes for different uses, such as cups, plates, deli trays, or as lids or covers for any of these common shapes.
Thermoformed plastic objects, such as lids or covers for deli containers (hereafter “lids”) are typically nested and stacked together as they come off the line at the end of a continuous feed manufacturing process. If the lids were all completely identical, they would nest together perfectly and engage their surfaces almost entirely. This level of engagement between adjacent lids can be problematic because the lids are made of relatively soft and hot plastic (having been just thermoformed at high temperature). If the lids stick together, that can cause manufacturing problems such as line stoppages and/or scrap.
To avoid the problems associated with perfect engagement between identical lids in a stack, lids have been made with convex external protrusions or lugs that differ for adjacent lids in the stack. This prevents the adjacent lids from nesting together, creating a vertical space between adjacent lids in the stack. The right amount of vertical space can keep adjacent lids in the stack from sticking together, it can aid outgassing and rapid and uniform cooling of the plastic lid at the end of the thermoforming process, and it can aid separability at the eventual point of use. However, too much space between adjacent lids can reduce efficiency in storage and transportation, so there is a tradeoff between tight stacking for efficiency and loose stacking for separability.
In practice, some consumers find the convex external protrusions or lugs to be unsightly. Also, some consumers find the apparent differences between different units of the same product to be confusing. What is needed is a thermoformed lid with improved and hidden features that enhance stacking, separability, and uniformity when multiple lids are stacked together after manufacture, for storage or transport prior to their final use or sale.